The subject matter of the present invention relates generally to optical data record recording and playback apparatus in which two or more light beams are employed for playback to simultaneously scan a corresponding number of data tracks recorded on an optical record. In particular the invention relates to such a playback apparatus in which closed loop optical servo means are employed to deflect the light beams during scanning for tracking the light beams along the data tracks and for maintaining the spacing between the two light beams to that corresponding to the spacing between adjacent tracks, as well as a time adjust means for adjusting the relative time or phase of two readout signals produced by such light beams during playback.
The playback apparatus of the present invention is especially useful for playing back optical records having high density data storage of analog or digital information in the form of closely spaced adjacent data tracks. For example, such an optical record can be used to store video and audio television signals.
Previously, it has been proposed in my earlier U.S. Pat. No. 3,501,586, granted Mar. 17, 1970, to provide a single light beam playback apparatus for optical records employing a light detector and mirror servo for maintaining the light beam on the data track during scanning. Similar playback apparatus are disclosed in U.S. Pat. No. 3,854,015 of Janssen, granted Dec. 10, 1974, and U.S. Pat. No. 3,673,412 of Olson, granted June 27, 1972, both of which employ a single light beam for tracking using a servo.
It has also been proposed to simultaneously playback two data tracks on an optical record by means of two light beams without the use of servos, as shown in U.S. Pat. No. 1,925,608 of Round, issued Sept. 5, 1933. However, the data tracks are of analog audio signals which are not closely spaced together so that mechanical alignment of the light beams in such patent is sufficient for proper tracking and beam spacing. A similar teaching is shown in U.S. Pat. No. 3,370,133 of Bender, granted Feb. 2, 1968. While mechanical adjustment can be employed to insure proper tracking and spacing between the readout light beams when the data tracks are spaced relatively far apart, it is not satisfactory when the data tracks are closely spaced from one another, such as on the order of 5.0 to 0.5 micron, as in the case of high density storage requiring high scanning speeds to produce output signals having data rates on the order of 50 to 100 megabits per second. Another problem with mechanical alignment is the difficulty in maintaining the time relationship between the two output signals produced by scanning adjacent data tracks during playback so that they are synchronized to within about 2 nanoseconds or 2 .times. 10.sup.-9 second. An even more serious problem results when the record is not dimensionally stable, causing track position and spacing to change due to heat expansion or wear so that the alignment problem would vary from record to record, which would make mechanical tracking of such high density storage records impossible.
The present invention overcomes the above discussed problems of the prior art by employing three independent servos. Two closed looped optical servos are provided for deflecting the light beams in response to photodetector sensed error signals to adjust the beams laterally with respect to the tracks and to adjust the spacing between such beams to maintain such beam spacing equal to the track spacing. A third closed loop optical servo or an electronic time delay is provided for adjusting the time relationship of the two light beams or their readout signals in order to synchronize the two readout signals of adjacent tracks.
It should be noted that variable electronic time delays have been employed previously to synchronize the signals of magnetic tape recorders, as shown in U.S. Pat. No. 2,842,756 of Johnson, granted July 8, 1958, and U.S. Pat. No. 3,327,299 of Johnson, granted June 20, 1967. However, neither of these patents relate to the problem of scanning two light beams along a pair of closely adjacent data tracks on an optical record, in the manner of the present invention.
The advantages of the multiple beam playback apparatus of the present invention are many, including reduced scanning speed during recording and playback, and reduced modulation rate for modulating the light beam during recording over that required for a single light beam recorder and playback apparatus. It enables closer spacing between tracks for higher density storage and maintains time relationship or synchronism between readout signals at high data rates. Finally, it automatically compensates for changes in position of the data tracks or spacing between the tracks due to dimensional instability of the recording or variations in the beam position or spacing between beams due to improper mounting and mechanical adjustment of the playback apparatus. This automatic compensation is achieved by simple and economical servos which operate in a fast and accurate manner over a long useful lifetime.